EP 0 330 733 A1 proposes a security thread that is testable both visually and by machine. For this purpose a transparent plastic foil is coated metallically and this coating provided with gaps in the form of characters or patterns. Furthermore, the security thread contains in the areas congruent with the gaps chromophoric and/or luminescent substances that cause the characters or patterns to differ in color contrast from the opaque metal coating under suitable light conditions. The metal layer used is preferably an aluminum layer. This security thread is embedded in security papers as a so-called “window security thread,” i.e. it is quasi woven into the paper during sheet formation of the security paper so that it is freely accessible on the surface of the paper at regular intervals and completely embedded in the paper only in the intermediate areas.
This security thread already meets a very high security standard. The continuous metallic coating permits machine testing of electric conductivity, while the gaps serve as a visual authenticity feature readily recognizable to the viewer in transmitted light. In addition, the thread has an additional feature not readily recognizable to the viewer, namely the luminescence in the area of the gaps, which is likewise testable by machine. When bank notes having such a security thread are viewed fleetingly, however, the metallic luster of the window areas is primarily striking. This luster can be imitated by simply gluing aluminum foil elements. Upon a fleeting check solely in reflected light, such forgeries could therefore be taken for authentic bank notes.